Hard to say for sure, but I would second the Northern Watersnake (sometimes also called a Common Watersnake). They get killed all the time around here by people that think they have Cottonmouths in their ponds, even though we are probably 500 miles from their typical range. You are probably right on the northernmost edge of the Cottonmouth territory. I don't doubt you get them in your area, but probably not very common.
